The lowest cost option would be something similar to this (I am not suggesting this particular model)

https://www.amazon.com/Battery-Daiwa-Electric-Fishing-Charger/dp/B01ABRX3YG/ref=asc_df_B01ABRX3YG/

You need to consider the battery charger as well.

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If you have any lithium battery charger at home or other battery chargers then the best battery for that may be different from the suggestion above